C++: Add an interprocedural control-flow library

[MathiasVP]

This adds a "shared" (i.e., language-independent) interprocedural control-flow library to the repository and instantiates it for C++.

Normally these things should be put under the shared folder, but I didn't do that as this would be the first "Microsoft-only" shared library in that folder. So let's just keep even the language-independent parts in the C++ folder for now.

Using this library is a lot like using the global dataflow library. Here's an example of how to use it:

/**
 * @kind path-problem
 */
import cpp
import semmle.code.cpp.interproccontrolflow.ControlFlow
import Flow::PathGraph

module Config implements ControlFlow::ConfigSig {
  predicate isSource(ControlFlow::Node source) {
    source.asParameter().hasName("argv") // start control-flow at the argv declaration
  }

  predicate isSink(ControlFlow::Node sink) {
    sink.getEnclosingFunction().hasName("foo") // and end at any node inside the "foo" function
  }
}

module Flow = ControlFlow::Global<Config>;

from Flow::PathNode source, Flow::PathNode sink
where Flow::flowPath(source, sink)
select sink.getNode(), source, sink, ""